1. Field of the Invention
The present invention relates to a curable composition that can provide a cured material having high hardness and a low level of cure shrinkage. The present invention also relates to a hardcoated article having a cured resin layer that is obtained by coating the above-mentioned curable composition on a substrate and curing it. More particularly, the present invention relates to a hardcoated article that has a low occurrence of coating peel-off and cracks, and has excellent abrasion resistance and surface hardness. Furthermore, the present invention relates to a hardcoated film that has a low occurrence of coating peel-off and cracks, is less curled after curing, and has excellent abrasion resistance and surface hardness.
2. Description of the Related Art
In recent years, plastic products have been superseding glass products because of ease of processing and reduction in weight. Since the surface of a plastic product is susceptible to damage, in many cases a hardcoat layer is formed directly thereon by coating a curable resin, or a hardcoat layered plastic film (called a xe2x80x9chardcoated filmxe2x80x9d) is laminated thereon with the aim of imparting abrasion resistance to the surface. Conventional glass products are also increasingly laminated with a plastic film in order to prevent scattering, and forming a hardcoat layer on the surface of such a film is useful and widely carried out so as to increase the hardness of the surface.
As conventional hardcoat materials, thermosetting coating materials and UV curable coating materials are used. However, for the purpose of obtaining hardcoated films, which are produced by coating the materials on plastic films and curing them, since the heat resistance of plastic films is poor, the UV curable coating materials, which can be cured at low temperature, are widely used. In general, curable components used in the UV curable coating materials are compounds called polyfunctional acrylate monomers, which have 2 to 6 acrylate ester groups in the molecule, and oligomers called urethane acrylates, polyester acrylates or epoxy acrylates, which have a molecular weight of a few hundred to a few thousand and several acrylate ester groups in the molecule. However, since most of the above-mentioned oligomers have a low acrylic group content and cannot achieve sufficient hardness by themselves, the polyfunctional acrylate monomers are widely used as hardcoat materials to give the required higher degree of hardness.
Generally, the hardcoated films are produced by coating the above-mentioned hardcoat materials as a thin coating of the order of 3 to 10 xcexcm on a plastic film, either directly or via a ca. 1 xcexcm primer layer. However, in conventional hardcoated films, since the hardness of the hardcoat layer is insufficient and the coating thickness thereof is low, when the base plastic substrate film deforms, the hardcoat layer deforms accordingly. The hardness of the entire hardcoated film is thus low and is not completely satisfactory. For example, a hardcoated film obtained by coating a UV curable coating material at the above-mentioned thickness on a cellulose triacetate or polyethylene terephthalate film, which are widely used as plastic substrate films, generally has a pencil hardness level of 2H to 3H, which falls far short of the pencil hardness of glass of 9H.
In order to compensate for the inadequate hardness, simply increasing the thickness of the hardcoat layer from the usual range of 3-10 xcexcm can improve the hardness of the hardcoated film. However, this causes the problems that the hardcoat layer so obtained easily cracks while at the same time the volume shrinkage during curing results in degradation of the adhesion to the substrate, thereby causing peel-off, and there is increased curling of the hardcoated film. It has therefore been difficult in the art to obtain a hardcoated film that has good characteristics and can be put into actual use.
JP-B-02-60696 (JP-B denotes Japanese examined patent application publication) (and the corresponding U.S. Pat. No. 4,427,823) discloses a coating composition containing a polyfunctional acrylate monomer as a resin-forming component for the hardcoat layer, and further containing a powdered inorganic filler such as alumina, silica or titanium oxide, and a polymerization initiator. JP-B-62-21815 (and the corresponding U.S. Pat. No. 4,624,971) discloses a photopolymerizable composition containing an inorganic filler made of silica or alumina whose surface has been treated with an alkoxysilane, etc. JP-A-2000-52472 (JP-A denotes Japanese unexamined patent application publication) proposes a method in which the hardcoat layer is formed from two layers, and adding particulate silica to the first layer improves curl and abrasion resistance. As described in these publications, the addition of inorganic particles improves the hardness of the hardcoat layers and suppresses the level of cure shrinkage, but does not satisfy the recent requirements for the hardcoat surface hardness.
JP-A-8-73771 discloses a photocurable hardcoat composition that is a mixture comprising a compound having at least 3 radically polymerizable groups in the molecule and a cationically polymerizable compound having 1 to 5 epoxy groups in the molecule. In an embodiment of this publication, although the cure shrinkage can be suppressed to some extent, a sufficient hardness cannot be obtained; increasing the coating thickness in order to enhance the surface hardness results in a high level of curl in the hardcoated film, and requirements in terms of the hardness and cure shrinkage are not satisfied.
JP-A-2000-71392 discloses a hardcoated film in which the hardcoat layer is formed from two layers, the lower layer being a cured resin layer formed from a blend of a radically curable resin and a cationically curable resin, and the upper layer being a cured resin layer formed from a radically curable resin alone. This publication describes setting the elasticity of the lower layer lower than that of the upper layer so that an applied stress can be absorbed by deformation of the hardcoat layer. In an example thereof the use of a monofunctional or bifunctional cationically curable compound in the lower layer actually suppressed the occurrence of coating peel-off, cracks and curling and achieved a pencil hardness of 4H. However, although such an arrangement can suppress the occurrence of coating peel-off and scratches on the coating, which are defined as damage in JIS K5400, the coating is very easily dented. Attempting to increase the hardness or the coating thickness of the upper layer in order to improve the denting generates cracks on the edge of the film when it is cut, and the recent requirements for the hardcoat surface hardness are not satisfied.
As a result of an intensive investigation by the present inventors, it has been found that the use of a specified compound in a curable composition can give a cured product that has high hardness and low cure shrinkage. It has also been found that by coating the curable composition on a substrate a hardcoated article can be obtained that is resistant to coating peel-off and cracks after curing, and has sufficient hardness. Furthermore, it has been found that, even when a plastic film is used as a substrate, a hardcoated film can be obtained that has less curl caused by cure shrinkage and has sufficient hardness.
The present invention has been carried out in view of the above-mentioned circumstances.
It is an object of the present invention to provide a curable composition that can give a cured material having high hardness and low cure shrinkage. It is another object of the present invention to provide a hardcoated article that is resistant to coating peel-off and cracks and has sufficient hardness. It is yet another object of the present invention to provide a hardcoated film which, even in the case where a plastic film substrate is used, is resistant to coating peel-off and cracks, avoids the problem of curl caused by cure shrinkage, and has sufficient hardness.
A first aspect of the present invention relates to a curable composition which is curable by polymerizing both a group capable of ring-opening polymerization and an ethylenically unsaturated group, wherein the group capable of ring-opening polymerization is present in a side chain of a cross-linkable polymer containing a repeating unit represented by formula (1) below. 
In the formula, R1 denotes hydrogen or an alkyl group having 1 to 4 carbon atoms, P1 is a monovalent group containing a group capable of ring-opening polymerization, and L1 is a single bond or a divalent linking group.
A second aspect of the present invention relates to a hardcoated article employing a curable composition which has been cured by polymerizing both a group capable of ring-opening polymerization and an ethylenically unsaturated group, wherein the group capable of ring-opening polymerization is present in a side chain of a cross-linkable polymer containing a repeating unit represented by the above-mentioned formula (1).
A third aspect of the present invention relates to an image display device equipped with a hardcoated film, the hardcoated film corresponding to the above-mentioned hardcoated article where a plastic film is used as the substrate.
The above-mentioned objects, other objects, features, and advantages of the invention will become clear from the following description.